1. Field of the Invention
This invention relates to a microwave oscillation circuit used in microwave radio equipment.
2. Description of the Related Art
A prior art microwave oscillation circuit is shown in FIG. 1. Collector electrode C of a transistor 1 is grounded. The base electrode B is connected to a transmission line 22 which is coupled with a resonator 21 which determines the oscillation frequency and also is connected to a resistor R.sub.1 via a capacitor C.sub.1 at the opposite end. Resonator 21 is typically formed of a dielectric resonator of TE.sub.01.delta. mode. Capacitor C.sub.1 is to block DC (direct current) current. The coupling via space L between the transmission line 22 and the resonator 21 is chosen so that the oscillation is sufficiently stable and includes minimum noises. A base bias voltage V.sub.B is supplied via a choke Ch.sub.2. The emitter is connected to an output stub, which is an open stub 3 of an appropriate length for the oscillation condition. An output transmission line 4 is placed in parallel to the output open stub 3 via an appropriate space therebetween. Output power of the oscillator is supplied from open stub 3 to the output transmission line 4. Emitter voltage V.sub.E is supplied via a second choke Ch.sub.1. An end of output transmission line 5 is open and another end P is connected to an input terminal Q of a buffer amplifier 5. If this connection is direct as shown by the numeral 41 in FIG. 1, the output impedance Z.sub.s looking at the output transmission line 4 from the input terminal Q is deviated far from the matched condition, such as very high impedance, because the opposite end is open. This mis-matached condition is likely to cause an unstable operation of buffer amplifier 5, such as a spurious oscillation at a very low frequency caused from increased amplification capability of the microwave transistor used in buffer amplifier 5. Therefore, in order to prevent this problem, there has been employed an isolator 42, an attenuator 43 or a 50-ohm series resistor 44 in place of the direct connection. The employment of 50-ohm series resistor 44 is disclosed in Japanese Tokukai Sho 64-62005 by the same inventor. However, in these prior arts, there is a problem in that and the additional device, such as isolator 42 or attenuator 43, is expensive, as well as the output power of the oscillator is attenuated to input to the buffer amplifier 5.